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Old 09-27-2012, 04:30 PM   #1
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AC 220 Shut off?


Here is my problem.
I have 3 phase power.
I have 6 HVAC units.
Each unit uses 2 phases of power (220) for the condenser unit, and 110v for the furnace control.
If I lose 1 phase of power, a unit can be turned on, and and condenser can be "half working" on 1 leg.
OR it can be working fine - it depends on which phase is out and how the units are wired in the panel.
What I would like is some kind of way to know when a phase is out and kill the units at the furnace control if it is affected.
Something that senses across 2 phases and can turn off a 110 circuit if either of the phases are missing.
Does anyone know of anything like that?

Thanks!!
Scott

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Old 09-27-2012, 04:34 PM   #2
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AC 220 Shut off?


Things like this can be custom built and wired. You won't find anything on a shelf somewhere.

Also, your A/C's will NOT run on one leg and be "half working. It's physically and electrically impossible.

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Old 09-27-2012, 07:17 PM   #3
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AC 220 Shut off?


I would build something with three 3-pole contactors. Put all the contactor poles in series, and connect each coil between one phase and neutral. If any phase goes out, it shuts down power on all three phases to the connected loads.
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Old 09-27-2012, 09:56 PM   #4
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AC 220 Shut off?


Quote:
Originally Posted by mpoulton View Post
i would build something with three 3-pole contactors. Put all the contactor poles in series, and connect each coil between one phase and neutral. If any phase goes out, it shuts down power on all three phases to the connected loads.

good idea !
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Old 09-27-2012, 10:17 PM   #5
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AC 220 Shut off?


Better yet, a revised plan: One contactor, and two small relays. One phase supplies power to the contactor coil, and the two relays complete the coil circuit only when the other two phases are hot. Much cheaper and smaller.
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Old 09-27-2012, 10:47 PM   #6
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AC 220 Shut off?


Why do you feel you need to do this? Just curious.
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Old 09-27-2012, 10:53 PM   #7
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AC 220 Shut off?


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Originally Posted by escottm View Post
Something that senses across 2 phases and can turn off a 110 circuit if either of the phases are missing.
Does anyone know of anything like that?

Thanks!!
Scott
Of course. They have been around for several years. We used them on all 3 phase HVAC to protect the components in the event of a phase loss. ANY hvac supply house should have them in some shape or form .3-Phase Monitor Phase Loss Single Phasing and Phase Protection Relay
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Old 09-27-2012, 11:19 PM   #8
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Why do you feel you need to do this? Just curious.
Single-phasing protection is pretty important for a lot of 3-phase equipment. But I'm sure you know that - you're an electrician. So I assume your point is that he states that these units are each single phase, and so phase loss shouldn't matter. That's a good point that a bunch of us glossed over. To the OP: Why, exactly, do you think you need phase loss protection for this equipment? Having the 120V control circuit live when the unit loses 208V power shouldn't hurt anything. Phase loss is only a problem on 3-phase gear.
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Old 09-28-2012, 07:05 AM   #9
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Originally Posted by escottm View Post
Here is my problem.
I have 3 phase power.
I have 6 HVAC units.
Each unit uses 2 phases of power (220) for the condenser unit, and 110v for the furnace control.
Guys, he is not talking about protecting 3-phase equipment. The condensing units are single-phase 240V, and he thinks if he looses a phase they will run on "half-power", which is NOT true.
This is why I originally asked WHY he wants/needs to do this.
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Old 09-28-2012, 07:24 AM   #10
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AC 220 Shut off?


I would be more concerned about the frequency he looses a leg that he feels it necessary to install such a monitor.
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Old 09-28-2012, 09:24 AM   #11
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Phase loss is only a problem on 3-phase gear.
Exactly why I asked.
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Old 09-28-2012, 10:45 AM   #12
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AC 220 Shut off?


I see some great ideas here.
I do want to clarify my application for those who asked.
The furnace controls are single phase - they are what calls for the AC compressor to come on in the seperate split systems.
The condensor units with cooling fans and compressors are 2 phase.
In the breaker panel, I have a 2 pole breaker for the outside condensor unit, but a single pole breaker for the inside furnace control unit.
If the wrong phase goes down, my controls will call for AC, but my compressor and fans will have lost one leg.
The compressor buzzes and the fan barely turns.
I am sure that isnt good for either.
That is why I would like to monitor the phases and if the outside condensor unit does not have both its phases, then I want to shut off the control.
However, it is just as well if I shut off both phases to the condensor - either way will work. Turn off the control, or turn off all power to the 2 phases.
Scott
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Old 09-28-2012, 10:50 AM   #13
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AC 220 Shut off?


Speedy,
I hear you, but what I am telling you is what I have observed.
When 1 phase goes down, and half the lights are off in my warehouse,
some of the AC is running normally and some are barely turning.
NOT dead, but barely turning and making unusual noises.

I am imagining a 220 motor trying to run on 110.
Is that not a thing?
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Old 09-28-2012, 11:52 AM   #14
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AC 220 Shut off?


Ya it will hum a bit and not work. I think what you should be fixing is your 3 phase power. In my years at trade working on 3 phase rarely does one phase ever drop out enough to worry about it.

You could get a contactor and a relay and wire a start stop circuit in so that if either phase looses power the contactor drops out. Still doesn't solve the main issue of why your loosing a leg.
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Old 09-28-2012, 12:31 PM   #15
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AC 220 Shut off?


Andrew,
my business is on a block that just happens to have intermittent problems. It always seems like the car hits the pole, or lightening strikes the transformer, or the circuit just plain old pops.
But it is the utility company, not me.
Not much to do about that.
I actually have a 3 phase Generator on the building, but i opted for a size that only runs 1 of my 2, 200 amp panels. So whatever is in the second panel is still exposed to losing a leg.

I am just trying to tie up "loose ends" as I have just replaced 2 motors and 1 compressor on the HVAC units, I wondered if there was a link between intermittent power problems and the life of my units ..... and if there was anythign to do about it.

Scott

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